Method of making polymer compositions containing thermoplastic starch

ABSTRACT

There is provided a method of preparing a thermoplastic starch and synthetic polymer blend, said method comprising the steps of: (a) providing a starch suspension comprising starch, water and a plasticizer, preferably glycerol; (b) obtaining a thermoplastic starch from the starch suspension by causing gelatinization and plasticization of said starch suspension by exerting heat and pressure on said starch suspension in a first extrusion unit; (c) evaporating and venting off residual water from said thermoplastic starch to obtain a substantially moisture-free thermoplastic starch; (d) obtaining a melt of a synthetic polymer or polymer blend in a second extrusion unit; and (e) combining the melt obtained from step (d) with the substantially moisture-free thermoplastic starch. Also provided are compositions of matter comprising immiscible blends of thermoplastic starches, polymers, and compatibilizers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] This invention relates to polymer compositions containingthermoplastic starch and to a method of making these compositions.

[0003] 2. The Prior Art

[0004] The blending of starch with synthetic polymers such aspolyethylene and polypropylene has been the subject of increasinginterest over recent years. The motivation is keen since starch is anabundant and inexpensive filler material. Moreover, starch may alsoimpart partial biodegradability to the resulting blend.

[0005] Natural starch found in plant products can be isolated as agranular powder. It is known that natural starch can be treated atelevated temperature and pressure with addition of defined amounts ofwater to form a melt. Such a melt is referred to as gelatinized ordestructurized starch. It is also known to mix destructurized starchwith additives such as plasticizers to obtain a thermoplastic starch orTPS. It is known to mix these forms of starch with synthetic polymersand co-polymers. For example, U.S. Pat. No. 5,095,054, and Ind. Eng.Chem. Prod. Res. Dev. vol 19, p. 592 (1980) describe such a process.

[0006] Difficulties have arisen in that the presence of starch has had anegative impact on the physical properties of the resulting mixture whencompared to the pure synthetic polymers. Furthermore, when starch ismixed with synthetic polymers or co-polymers, the starch domains areenveloped by the non-biodegradable synthetic polymers and consequentlytheir biodegradability is significantly reduced.

[0007] A biodegradable material can be defined as one that is able to beconverted to CO₂ and H₂O by certain common microorganisms.

[0008] It is further unknown in the art to achieve mixtures of starchwith non-biodegradable polymers where the starch domains are readilyaccessible for environmental degradation while still maintaining goodmechanical properties.

[0009] With respect to the method of preparing polymer and TPS blends,some blending studies have been reported using internal mixers. Examplesof such studies are found in international application WO 90/14388,European Patents 0 554 939 and 0 327 505.

[0010] It is also known from the article entitled “Processing andcharacterization of thermoplastic starch/polyethylene blends”, publishedin Polymer, 38 (3), 647 (1997), to blend TPS and low densitypolyethylene (LDPE) in a continuous process using a co-rotatingarrangement of a twin-screw extruder fed on one side by a single-screwextruder. The side extruder is used to prepare the TPS. The mainextruder is used to prepare the LDPE melt which is combined with the TPSmelt. However, such process results in TPS/LDPE blends having poorphysical properties including the presence of water and of bubbles.Moreover, tensile properties of the extrudate dropped off dramaticallyat about 10% or more of TPS content. Tests revealed that the TPS,present as a dispersed phase in the extrudate, exhibited spherical orellipsoidal shapes. Consequently, the extrudate was not easilybiodegradable since the great majority of spherical or ellipsoidalshapes were enveloped in polyethylene which is not biodegradable. Inother words, the dispersed TPS phase was not continuous.

[0011] The prior art is also silent on controlling process parameters toachieve controlled morphologies of the resulting blend.

[0012] Thus, it is an object of the present invention to provide a novelmethod for obtaining TPS/polymer blends having controllable and improvedphysical properties over the prior art blends.

[0013] It is a further object of the present invention to provide animproved product comprising a blend of TPS and polymer(s) havingimproved physical properties over prior art blends.

[0014] It is a related object of the invention to provide an improvedproduct wherein the TPS phase is continuous so as to allowbiodegradation processes to take place within the product.

[0015] In preferred embodiments, the product is extruded sheet or blownfilm.

[0016] Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. It should be understood, however, that this detaileddescription, while indicating preferred embodiments of the invention, isgiven byway of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art.

SUMMARY OF THE INVENTION

[0017] There is provided a method of preparing a thermoplastic starchand synthetic polymer blend, said method comprising the steps of: (a)providing a starch suspension comprising starch, water and aplasticizer, preferably glycerol; (b) obtaining a thermoplastic starchfrom the starch suspension by causing gelatinization and plasticizationof said starch suspension by exerting heat and pressure on said starchsuspension in a first extrusion unit; (c) evaporating and venting offresidual water from said thermoplastic starch to obtain a substantiallymoisture-free thermoplastic starch; (d) obtaining a melt of a syntheticpolymer or a polymer blend in a second extrusion unit; and (e) combiningthe melt obtained from step (d) with the substantially moisture-freethermoplastic starch.

[0018] Also provided are compositions of matter comprising immiscibleblends of thermoplastic starches and polymers. The compositions ofmatter exhibit favorable mechanical properties and provide a continuousor highly continuous thermoplastic phase so as to enhance thebiodegradability of the composition of matter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a schematic illustration,top view, of the apparatus used in a preferred embodiment of the methodof the present invention;

[0019]FIG. 1B is a side view;

[0020]FIGS. 2a)-2 d) are scanning electron micrographs (SEM) of variousembodiments of the product of the present invention wherein the productis an LDPE/TPS blend (ca. 30% TPS): (a) LDPE2040/TPS (20% glycerol)500X; (b) LDPE2049/TPS (20% glycerol) 500X; (c) LDPE2040/TPS 27.5%glycerol) 1000X; (d) LDPE2049/TPS (27.5% glycerol) 500X;

[0021]FIGS. 3a)-3 d) are scanning electron micrographs (SEM) of variousembodiments of the product of the present invention wherein the productis an LDPE2040/TPS (27.5% glycerol) blends cryogenically fractured inthe axial direction: (a) 29% 1000X; (b) 35.5% 1000X; (c) 44.7% 1000X;and (d) 53.3% 500X;

[0022]FIGS. 4a)-4 d) are scanning electron micrographs (SEM) of variousembodiments of the product of the present invention wherein the productis an LDPE2040/TPS (27.5% glycerol) blends cryogenically fractured inthe transversal direction: (a) 29% 1000X; (b) 35.5% 1000X; (c) 44.7%1000X; and (d) 53.3% 500X;

[0023]FIG. 5 is the accessibility of starch domains in LDPE/TPS blendsafter 96 hours of extraction;

[0024]FIG. 6a)-6 b) is the relative elongation at break (ε/ε₀) ofLDPE/TPS blends: (a) LDPE2040; (b) LDPE2049;

[0025]FIG. 7a)-7 b) is the relative Young's Modulus (Ε/Ε0) of LDPE/TPSblends: a) LDPE2040; b) LDPE2049;

[0026]FIG. 8 is the relative Young modulus (Ε/Ε₀) of LDPE2040/TPS blends(27.5% glycerol in slurry);

[0027]FIG. 9 is the relative elongation at break of LDPE2040/TPS blends(27.5% glycerol in slurry);

[0028]FIG. 10 is the relative engineering maximum strength (smax/smax0)of LDPE2040/TPS blends (27.5% glycerol in slurry).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] Before describing the present invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of construction and parts illustrated in the accompanyingdrawings and described herein. The invention is capable of otherembodiments and of being practiced in various ways. It is also to beunderstood that the phraseology or terminology used herein is for thepurpose of description and not limitation.

[0030] In general terms, the present invention provides novelcompositions of matter including TPS/synthetic polymer blends havingimproved physical properties including tensile properties and increasedTPS accessibility over prior art achievements. The inventionconcurrently provides a novel process for achieving the newcompositions.

[0031] As used herein, the expression starch refers to any starch ofnatural origin whether processed, chemically modified or treated,including starches such as for example: wheat starch, corn starch,potato starch, and rice starch. Starch can also be derived from plantsources such as cassava, tapica, and pea. It is a polysaccharide thatconsists essentially of a blend of amylose and amylopectin.

[0032] Starch includes modified starches, such as chemically treated andcross-linked starches, and starches in which the hydroxyl groups havebeen substituted with organic acids, to provide esters or organicalcohols to provide ethers, with degrees of substitution in the range0-3.

[0033] Starch includes extended starches, such as those extended withproteins; for example with soya protein.

[0034] As used herein, the expression synthetic polymer refers to anysubstantially water-insoluble synthetic thermoplastic or thermosetmaterials. Examples of substantially water-insoluble thermoplastichomopolymer resins are polyolefins, such as polyethylene (PE),polypropylene (PP), polyisobutylene; vinyl polymers, such as poly (vinylchloride) (PVC), poly (vinyl acetate) (PVA), poly (vinyl carbazoles);polystyrenes; substantially water-insoluble polyacrylates orpolymethacrylates, such as poly (acrylic acid) esters, poly (methacrylicacid) esters; polyacetals (POM); polyamides, such as nylon6, nylon-6,6,aliphatic and aromatic polyamides; polyesters, such as poly(ethyleneterephthalate) (PET), poly(butylene terephthalate) (PBT);polyarylethers; polyurethanes, polycarbonates, polyimides, and highmolar mass, substantially water-insoluble or crystallizablepoly(alkylene oxides), such as poly(ethylene oxide), poly(propyleneoxide). As well as mixtures thereof.

[0035] Further included are polyesters and polylactides that areconsidered biodegradable in short time periods. Examples of those waterinsoluble materials are polylactones such as poly(epsilon-caprolactone)and copolymers of epsilon-caprolactone with isocyanates; bacterialpoly(hydroxyalkanoates), such aspoly(hydroxybutyrate-3-hydroxyvalerate); and polylactides, such aspoly(lactic acid), poly(glycolic acid) and copolymers comprising therepetitive units of both; as well as mixtures thereof.

[0036] Further included are substantially water-insoluble thermoplasticalpha-olefin copolymers. Examples of such copolymers are alkylene/vinylester-copolymers as ethylene/vinyl acetate-copolymers (EVA),ethylene/vinyl alcohol-copolymers (EVAL); alkylene/acrylate ormethacrylate-copolymers preferably ethylene/acrylic acid-copolymers(EAA), ethylene/ethyl acrylate-copolymers (EEA), ethylene/methylacrylate-copolymers (EMA); alkylene/maleic anhydride-copolymerspreferably ethylene/maleic anhydride-copolymers; as well as mixturesthereof.

[0037] Further included are styrenic copolymers, which comprise random,block, graft or core-shell architectures. Examples of such styreniccopolymers are alpha-olefin/styrene-copolymers preferably hydrogenatedand non-hydrogenated styrene/ethylene-butylene/styrene copolymers(SEBS), styrene/ethylene-butadiene copolymers (SEB); styreneacrylonitrile copolymers (SAN), acrylonitrile/butadiene/styrenecopolymers (ABS); as well as mixtures thereof.

[0038] Further included are other copolymers such as acrylic acidester/acrylonitrile copolymers, acrylamide/acrylonitrile copolymers,block copolymers of amide-esters, block copolymers of urethane-ethers,block copolymers of urethane-esters; as well as mixtures thereof.

[0039] Further included are thermoset resins such as epoxy,polyurethane, polyesters and their mixtures.

[0040] Further included are block or graft copolymers formed betweenhomopolymers and copolymers of hydroxyacids and one or more of thefollowing components:

[0041] (i) Cellulose or modified cellulose.

[0042] (ii) Amylose, amylopectin, or natural or modified starches.

[0043] (iii) Polymers resulting from the reaction, of a compoundselected from diols, prepolymers or polymers of polyesters havingterminal diol groups with monomers selected from the group consistingof: bifunctional aromatic or aliphatic isocyanates; bifunctionalaromatic or aliphatic epoxides; dicarboxylic aliphatic acids;dicarboxylic cycloaliphatic acids; or aromatic acids or anhydrides.

[0044] (iv) Polyurethanes, polyamides-urethanes from diisocyanates andamino-alcohols, polyamides, polyesters-amides from dicarboxylic acidsand amino-alcohols, and polyester-ureas from aminoacids and diesters ofglycols.

[0045] (v) Polyhydroxylate polymers;

[0046] (vi) Polyvinyl pyrrolidone, polyvinyl pyrrolidone-vinyl-acetatecopolymers and polyetheloxazoline. As well as mixtures thereof.

[0047] In the method and product of the present invention, the additionof compatibilizers or coupling agents can also be envisaged.Compatibilizers improve the adhesion at the interface and can beparticularly useful at further improving the properties at high loadingsof thermoplastic starch. The addition of an interfacial modifier stillallows for the obtention of highly continuous and co-continuous networkshowever the scale of said network becomes substantially finer.

[0048] Suitable compatibilizers or coupling agents for the TPS basedblends can be polymers or copolymers having functional groups thatpresent specific interactions with starch molecules and/or are capableof undergoing chemical reactions with starch functional groups to resultin a chemical bond. Those compatibilizers have preferably a lowinterfacial tension with the synthetic polymer, but more preferably apartial or full miscibility with the synthetic polymer. Examples offunctional groups that present specific interactions and/or are capableto react with starch are: Hydroxyl, carboxyl or carboxylate, tertiaryamino and/or quaternary ammonium, sulfoxyl and/or sulfoxylate groups,and vinyl pyrrolidone copolymers.

[0049] The compatibilizer having hydroxyl groups is preferably a polymercontaining vinyl alcohol units. More preferably it is a poly (vinylester) wherein the ester groups are partially hydrolyzed or a copolymercontaining vinyl alcohol units as well as other units such as areobtained by copolymerization of vinyl esters, preferably vinyl acetate,with monomers such as ethylene (EVOH), propylene, vinyl chloride, vinylethers, acrylonitrile, acrylamide, omega-octadecene, vinyl-butyl ether,vinyl-octadecyl ether, vinyl pyrrolidone and other known monomers, withsubsequent hydrolysis of at least some of the vinyl-ester groups.Preferred copolymers are e.g. poly (vinyl alcohol-co-vinyl-acetate);

[0050] ethylene/vinyl alcohol/vinyl acetate copolymers; ethylene/vinylchloride/vinyl alcohol/vinyl acetate graft copolymers; vinylalcohol/vinyl acetate/vinyl chloride copolymers; vinyl alcohol/vinylacetate/vinyl chloride/diacryl amide copolymers; vinyl alcohol/vinylbutyral copolymers; vinyl alcohol/vinyl acetate/ vinyl pyrrolidonecopolymers; vinyl alcohol/styrene copolymers.

[0051] The compatibilizer containing carboxylic acid and/or carboxylategroups is preferably a synthetic polymer, preferably a copolymercontaining carboxylate groups as well as other units such as areobtained by copolymerization of acrylic acid, methacrylic acid, crotonicacid, maleic acid, itaconic acid, e.g. in their acid or carboxylateform, with monomers such as ethylene, vinyl chloride, vinyl esters suchas vinyl acetate, vinyl ethers, acrylic acid esters, acrylonitrile,methacrylic acid esters, maleic acid esters, acrylamide,omega-octadecene, vinyl-butyl ether, vinyl pyrrolidone and other knownmonomers. If a carboxyl group-containing monomer is used for preparingthe polymer, then at least a part of the carboxyl groups must beneutralized with a cation. Preferred copolymers containing carboxylategroups are those which can be described as being derived from acrylicacid, methacrylic acid, crotonic acid, maleic acid, itaconic acid,methylacrylate, methylmethacrylate, acrylamide, acrylonitrile and/ormethylvinylether. More preferred polymers are those that can bedescribed as being derived from acrylic acid, methacrylic acid, maleicacid, methacrylate, ethyl acrylate and/or methylvinylether. Suchcopolymers may be also copolymerized with ethylene, propylene, orstyrene. Such copolymers are, e.g., poly (acrylic acid-co-vinylacetate); ethylene/acrylic acid/vinyl acetate copolymers; ethylene/vinylchloride/acrylic acid/vinyl acetate graft copolymers; acrylic acid/vinylacetate/vinyl chloride copolymers; acrylic acid/vinyl methylethercopolymers; vinyl acetate/acrylic acid/acrylic acid methylestercopolymer; vinyl acetate/crotonic acid copolymers; vinyl acetate/maleicacid copolymers; methacrylic acid/vinyl acetate/vinyl pyrrolidonecopolymers; acrylic acid/acrylonitrile copolymer;ethylene/propylene/acrylic acid copolymer; and styrene/acrylic acidcopolymer, wherein a part or all of the acid groups are present in theircarboxylate form. Copolymers that contain carboxylic groups arepreferably copolymer of acids with ethylene, e.g. theethylene-acrylic-acid copolymer in the form of its salt or anethylene-methacrylic acid copolymer in the form of its salt.

[0052] Compatibilizers which contain tertiary amino groups and/or saltsthereof and/or quaternary ammonium groups are preferably a syntheticpolymer, as obtained by the polymerization of monomers containingtertiary amino groups and/or salts thereof and/or quaternary aminogroups such as poly (2-vinyl pyridine); poly (4-vinyl pyridine);polyvinyl carbazole, I-vinyl imidazole and/or salts thereof and/or theirquaternized derivatives as well as with other polymers as are obtainedby copolymerization of such amines with other monomers such asacrylonitrile, butyl methacrylate, styrene and other known monomers. Theexpression amine salts includes the salts formed with an inorganic ororganic acid, e.g. salts with inorganic or organic acids such as HC1,H2SO4, and acetic acid. The expressions “quaternized derivative” and“quaternary ammonium groups” mean quaternized derivatives of tertiaryamines, e.g. quaternized with an alkyl halide such as methyl chloride.Preferred polymers are those derived from 2-vinyl-pyridine, 4-vinylpyridine and vinyl carbazole.

[0053] Compatibilizers having sulphonic acid and/or sulfonate functionalgroups are preferably styrene sulphonic acid polymers, styrene sulfonicacid copolymers, and salts thereof. More preferably they are blockcopolymers of sulfonated styrene with unsaturated monomers such asethylene, propylene, butylene, isobutylene, butadiene, isoprene, and/orstyrene. Preferred salts thereof, including the corresponding sulfonatesare their salts with metal ions or the ammonium ion, preferably analkali metal ion, magnesium or zinc or NH₄ ⁺, preferably sodium,potassium or zinc, preferably the sodium salt.

[0054] Compatibilizers containing vinyl pyrrolidone are preferablycopolymers of vinyl pyrrolidone with one or more monomers selected fromthe group of vinyl esters, vinyl alcohol, allyl alcohol, ethylene,propylene, butylene, isoprene, butadiene, styrene, vinyl ethers, anddimethylaminoethyl methacrylate. Preferred are copolymers of vinylpyrrolidone with a monomer selected from the group consisting of vinylesters, vinyl alcohol, styrene and dimethylaminoethyl methacrylate.Preferred are further the poly (N-vinyl pyrrolidone-vinyl ester)copolymers and from these the poly (N-vinyl pyrrolidone-vinyl acetate)copolymers.

[0055] As used herein when referring to immiscible TPS/polymercompositions, the term “continuous” refers to either the TPS or thepolymer phase being essentially constituted of a network ofinterconnected domains. The term “co-continuous” refers to a compositionwherein both the TPS and the polymer phase are continuous. Theexpression “highly continuous TPS phase” refers to a composition wherethe TPS phase is dispersed in the polymer phase and yet the TPS domainsare nearly all interconnected. Highly continuous can be defined as thecase where 50% or more of the TPS is extractable. The per-centextractable TPS is based on the weight loss of TPS from a 1 mm length(machine-direction)×7.5 mm width (cross-direction) specimen subjected tohydrolytic degradation in a solution of HCl at 60 degrees Celsius for96-150 hours. Extracted samples were vigorously washed with distilledwater and dried at 60 degrees Celsius in a vacuum oven for 48 hoursprior to weight measurement. The concept of continuity of the TPS phaseis of particular importance when measuring the biodegradability of amaterial. If the TPS phase is not continuous or highly continuous, theTPS domains will be encapsulated by a non-degradable polymer renderingthe majority of the TPS phase substantially less accessible tobiodegradation.

[0056] As used herein, the term “plasticizer” refers to any suitableplasticizer for achieving a TPS. Plasticizers include for example:adipic acid derivatives, such as tridecyl adipate; benzoic acidderivatives, such as isodecyl benzoate; citric acid derivatives, such astributyl citrate; glycerol itself and derivatives;

[0057] phosphoric acid derivatives, such as tributyl phosphate;polyesters; sebacic acid derivatives, such as dimethyl sebacate; urea.

[0058] The plasticizer can be selected from the group consisting ofglycerin, ethylene glycol, propylene glycol, ethylene diglycol,propylene diglycol, ethylene triglycol, propylene triglycol,polyethylene glycol, polypropylene glycol, 1,2-propanediol,1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,2,6-hexanetriol,1,3,5-hexanetriol, neopentyl glycol, trimethylol propane,pentaerythritol, sorbitol, and the acetate, ethoxylate, and propoxylatederivatives thereof.

[0059] Moreover, the plasticizer can be selected from the groupconsisting of sorbitol ethoxylate, glycerol ethoxylate, pentaerythritolethoxylate, sorbitol acetate, and pentaerythritol acetate.

[0060] The plasticizer is present in a amount of from 1% to 50 wt %, butpreferably 5% to 50 wt % and most preferably 100% to 35 wt % calculatedon the total weight of the final composition. When expressed in term ofwt % in the initial slurry, the plasticizer was present in a proportionof about 10 to about 32 wt %.

[0061] All the references to % glycerol in the reported data refer tothe amount of glycerol in the initial slurry (TABLE 1). In the casewhere water is substantially eliminated from the TPS, the % glycerol (orany other plasticizer) in the blend material after extrusion can becalculated from the slurry concentrations in TABLE 1 based on thefollowing calculation. For example, in a final extruded blend productcontaining 53% LDPE/47% TPS prepared from a 48.5% starch/27.5%glycerol/24% water suspension, the per-cent glycerol in the extrudate isabout 19%. If water is present at substantial levels in the final blendproduct, its weight will also need to be taken into account.

Description of the Method of Preparing the Novel Compositions

[0062] The method of the present invention uses a starch suspension as afirst feed material and a synthetic polymer as a second feed material.The synthetic polymer is preferably ground into granules for ease ofmelt processing through a screw-type blender-extruder.

[0063] Referring now to FIGS. 1 and 2, there is shown a preferredembodiment of the extrusion apparatus used to carry-out the method ofthe invention. Referring to FIG. 1a, an upper view of the extrusionsystem 10 shows a twin-screw extruder (TSE) 12 to which is attached asingle-screw extruder (SSE) 14. In sharp contrast with the prior art,the thermoplastic starch (TPS) is prepared in the TSE 12 while thesynthetic polymer, in this case low density polyethylene (LDPE), ismelted in SSE 14. This method will be further described hereinbelow.

[0064] Preparation of the Starch Suspension

[0065] Wheat starch was mixed in different proportions with water andglycerol. During the starch extrusion, water is important to promote thegelatinization process. Once gelatinized, the glycerol plasticizesstarch. In addition to plasticizing starch; glycerol decreases theviscosity of TPS. In the suspension, the starch content varied from 48to 50% by weight. Water and glycerol were varied from 20% to 30% andfrom 32% to 19% by weight, respectively. The glycerol concentration wasvaried in order to achieve TPS of varying and controllable viscosities.The water content was modified to maintain a constant liquid/solid ratioof about 1:1 v/v. Three examples are reported in Table 1 below. Allcontents are expressed in terms of %/wt of suspension. TABLE 1 GlycerolExample Starch content* content* Water content* 1 48 32 20 2 48.5 27.524 3 50 20 30

[0066] In a typical suspension, 640 g of glycerol was mixed with 400 gof distilled water and placed in a recipient. 960 g of starch powder waspoured in the recipient containing water and glycerol and stirred togive a homogeneous slurry. The slurry, once made, was used immediatelyin the preparation of LDPE/TPS blends. Starch suspensions aresusceptible to the problem of sedimentation. Furthermore, the viscosityof the starch suspension increases with time. This increase has beenattributed to the solvation of starch molecules and furtherre-arrangement into a gel-like structure. For these reasons the starchsuspension must be used as fresh as possible, especially if theviscosity affects the feeding rate.

[0067] Feeding the mixture to the extruder as a slurry is a novelapproach to preparing these materials and ensures that the starch isfully destructurized and that the glycerol is well dispersed throughoutthe starch material. Both of those elements are necessary components toachieving blends with the high elongational properties achieved by thepresent invention.

[0068] One-Step Extrusion Process

[0069] a) Basic Setup

[0070] Blending was carried out in a one-step process. A single-screwextruder (SSE) 14 was connected to an intermediate zone of a co-rotatingtwin-screw extruder (TSE) 12 using a leak-proof adapter. The schematicrepresentation of the upper and side views of the extrusion system areshowed in FIGS. 1 and 2, respectively. This one-step approach allows forthe melt-melt mixing of the components which improves the morphologycontrol of the dispersed TPS phase. It also provides the possibility ofminimizing the contact time between the two polymers at high temperaturewhich is the principal parameter for controlling the thermal degradationof TPS. The single screw used was from C. W. Brabender Instruments(LID=26, length=495 mm, and compression ratio=2). The twin-screw was aLeistritz AG (LM 30.34), LID=28, and length=960 mm. The above describedsetup allows for the separation of the different processes occurring inthis operation. Accordingly, the melting of LDPE takes place in SSE 14,while both the starch gelatinization and plasticization (SGP) and meltblending occur in TSE 12. The mixing of TPS and PE occurs in the latterhalf of TSE 12. For ease of description, TSE 12 is pictorially dividedinto zones 16 to 30 as the blending progresses.

[0071] b) TPS Preparation

[0072] An important feature of the present method is the preparation ofthe TPS which comprises the steps of starch gelatinization andplasticization (SGP). The screw configuration in TSE 12 was chosen togive a long enough residence time, which permits SGP in the first zonesof TSE 12. SGP took place over three sub-sections of TSE 12: feedingsection 16, SGP sections 18 and 20 and water extraction section 22. Thestarch suspension was fed at a temperature lower than 25° C. in thefirst section of TSE 12. This zone was water-cooled in order to maintaina low temperature. SGP was carried out in the sections 18 and 20 of theTSE 12. Due to the thermal instability of starch, SGP was carried out at70° and 90° C. in the sections 18 and 20, respectively. Several kneadingsections were used to homogenize the resulting TPS. Back-flow kneadingelements were also adapted to increase the residence time and,consequently, ensure the complete destructuring and the homogeneity ofthe TPS. It also served to increase the extrusion pressure before theventing zone 22. Water extraction took place in section 22 of TSE 12.Low-pressure elements, a higher temperature (110° C.) and vacuum werefound to improve the water extraction. The venting zone 22 was connectedto a condensation system, which avoided the passage of volatiles throughthe vacuum line. Once the TPS is substantially water-free, it can beblended with the second polymer, in this case LDPE.

[0073] The flow rate of the extruded TPS had an influence on thepressure exerted by the starch and its final appearance. In order tostudy this phenomenon, an TSE extruder configuration using just fivezones was used. This configuration was similar to the original eightzones configuration, but zones 24, 26, 28 and 30 were taken out. Threecapillary dies were used to measure the viscosity of TPS. The flow rateof the starch suspension was compared to that of TPS at the exit of thecapillary die. Surprisingly, the difference between both flow rates wasalmost equal to the water content in the starch suspension. Likewise,TGA measurements indicated that the water content in TPS was around 1%.This approach is thus very effective in removing the water fromthermoplastic starch. This is a critical point since excess water givesrise to bubbles in the resulting starch/polymer blend. These bubbles notonly affect aesthetics but also diminish the mechanical properties ofthe blend. As such, TPS will be considered as a binary system composedof starch and glycerol.

[0074] In studying the effect of flow rate of the starch suspension onthe quality of the extrudate, lower and upper limits of feeding werefound. The lower limit was imposed by the increased residence time ofthe TPS. It is well known that the TSE works better under starve-fedconditions. In such a situation, the residence time is controlled by thescrew configuration, the flow rate and the screw speed. The screw speedwas maintained constant at 150 rpm in the whole series of melt mixingand viscosity measurement experiments. Evidence of degradation was foundat flow rates of the extruded TPS lower than 20 g/min. The appearance ofTPS changed from a transparent and flexible material to a yellowish morerigid one. When the flow rate of TPS was lower than the mentioned limit,an unexpected increase in the pressure was also monitored. At higherflow rates, the pressure was proportional to the measured flow rate. Theupper limit for the flow rate of TPS was imposed by the water extractionin the venting zone 22. Problems of foaming were observed at flow ratesbetween 45-50 g/min of TPS. In contrast to the lower limit, the pressureexerted by the foamed TPS decreased as the flow rate increased. Bothphenomena were produced by the presence of water in the extrudate. Watervapor, at 150° C. was responsible for the foaming of TPS. Moreover,water excess reduced the viscosity of TPS in the extruder. This upperlimit can be overcome by the addition of another venting zone or themodification of the existing one with more efficient equipment. As ismentioned above, the flow rate, temperature, and screw design areimportant parameters to control.

[0075] c) Mixing

[0076] The blend mixing section can be divided into three sub-sections:LDPE addition zone 24 mixing zone 26 and 28 and pumping zone 30. Thetemperature of the whole mixing section was maintained constant at 150°C. As observed in FIG. 1a, the LDPE addition zone 24 has no heatingelement, however, the temperature was maintained around 150° C. by theconvection heating of the neighboring zones 14 and 26 and the moltenLDPE. The melt mixing of LDPE and TPS starts in zone 24. The melt mixingcontinued through the next two zones 26 and 28 aided by several kneadingand mixing elements. The pumping zone 30 is necessary to pressurize theextrudate through the die head.

[0077] The proportion of thermoplastic starch in terms of wt % of theresulting TPS/polymer blend was about 10 to 60 wt %, and preferablyabout 20 to 55 wt %.

[0078] It is to be noted that by attaching the single-screw extruder 14progressively downstream (zones 26, 28 or 30) on the twin-screw 12 it ispossible to achieve the same level of morphology control as reportedhere at very low blend residence times. Thus, one of the advantages ofthe single step approach is that it can be used to minimize theresidence time of starch in contact with a high melting polymer.Therefore, TPS can be blended with high melting temperature polymerssuch as PP, PS, PET etc. while still minimizing thermal degradation ofthe starch.

[0079] The die head 32 and SSE 14 were operated at the same temperatureas the mixing section. The screw speed of SSE 14 was kept constant usingan arbitrary measure of the motor speed (2.5) and the flow rate of LDPEwas controlled with the aid of a pellet feeder. Maximum pumping of SSE14 under these conditions was 100 g/min.

[0080] d) Sheet Take-Up

[0081] LDPE/TPS blends were extruded through a rectangular die. Blendswere quenched using calendar rolls. Calendar rolls were used becauseblends could not be quenched in cold water due to the highly hydrophilicnature of TPS. The strain ratio, the ratio between the speed ofextrudate and the speed of the ribbon at the exit of the calendar, wasaround 2. That imposed a machine direction deformation on the ribbon.The morphology of those blends showed evidence of that deformation. Theevolution of the morphology of LDPE/TPS blends are reported below.

Novel TPS/Polymer Compositions

[0082] The morphology of LDPE/TPS blends prepared in accordance with themethod of the present invention was studied using a scanning electronmicroscope (SEM). LDPE/TPS blend ribbons were cryogenically fractured toobtain surfaces both axial and transversal to the machine direction.Fractured samples were coated with gold palladium alloy and furtherobserved in a JSM-820 SEM.

[0083] a) Influence of the Glycerol Content

[0084] Micrographs taken in the axial direction (machine direction) ofPE blended with ca. 30% of TPS and compounded with either 20% or 27.50%of glycerol is shown in FIG. 2. The particle diameter of TPS domainscompounded with 20% glycerol was larger than that of JPS having higherglycerol content.

[0085] Furthermore, TPS (20% glycerol) domains demonstrated only aslight deformation even though all blends were quenched at similartake-up speeds. This could be the consequence of the higher viscosity ofTPS compounded with only 20% glycerol. The viscosity of polymerscompounded with low molecular weight plasticizer decreased as theplasticizer content increased. Particles of TPS made with 20% glycerolwere elliptical with a minor axis diameter ranging between 10 um to >50um. That means that those particles were larger than the originalgranular starch. This is surprising considering that TPS has beencompletely gelatinized and plasticized. It seems that the viscosity ofthe two LDPE types tested was not high enough to disintegrate TPSparticles containing 20% glycerol into a smaller dispersed phase.However, blends prepared with LDPE2040, the PE having the lower meltflow index, demonstrates a finer particle size than that of LDPE2049. Onthe other hand, the TPS compounded with the larger quantity of glycerolwas deformed into fiber particles by both PIE matrix materials.

[0086] b) Influence of the LDPE/TPS Concentration Ratio

[0087] The evolution of the TPS domains as a function of composition inLDPE/TPS blends in the axial direction is shown in FIG. 3. It isimportant to note that the glycerol content (based on the slurry) inthis TPS was 27.5%. The fiber-like structures (found throughout thethickness of the sheet) are a result of the high concentration of TPSand are also due to deformation processes experienced in the die and asthe material exits the die. This structure is preserved by quenchingcalendar rolls. The fiber diameter increased from 2-4 um to >10 um asthe concentration of TPS increased from 29% to 53.5% TPS. TPSfiber-fiber coalescence is evident at TPS concentrations of 35.5% ormore. The morphology of LDPE/TPS blends fractured in the transversedirection revealed that TPS domains were more strongly deformed in themachine direction, see FIG. 4. As observed in the axial viewmicrographs, the fiber diameter increased as the TPS concentrationincreased. However, evidence of coalescence was observed even at thelowest TPS concentration (29.0%). Coalescence of the TPS domainsoccurred to a very high degree at 53.3% TPS.

[0088] It is possible, if desired, to form a thin layer of LDPE at thesurface of the product.

Accessibility of Thermoplastic Starch

[0089] Starch-based materials require that two important, and closelyrelated, aspects be controlled: water absorption and biodegradability.Water permits the microorganisms to move and also helps them tometabolize starch. Nevertheless, water may also affect the dimensionalstability of starch-based materials and their properties. The presentinvention tackles this problem by controlling the morphology of theseblends. The continuous structure allows for the accessibility of starchdomains. The accessibility of starch domains in LDPE2040/TPS blends wasstudied. The percent extractable TPS is based on the weight loss of TPSfrom a 1 mm length (machine-direction)×7.5 mm width (cross-direction)specimen subjected to hydrolytic degradation in a solution of HCl at 60degrees Celsius for 96-150 hours. Extracted samples were vigorouslywashed with distilled water and dried at 60 degrees Celsius in a vacuumoven for 48 hours prior to weight measurement.

[0090] Blends of LDPE/TPS having higher glycerol contents showed afiber-like and nearly continuous morphology in the machine direction.Consequently, a higher accessibility in the axial direction wasexpected. In order to determine the influence of such connectivity ondegradability, samples were exposed to hydrolytic extraction, see FIG.5.

[0091] In both, LDPE2049/TPS (20% glycerol) and LDPE2040/TPS (27.5%glycerol), the accessibility of starch domains increases with TPSconcentration and reaches a maximum at the phase inversion region. TPScontaining a high glycerol content of about 27.5 wt % was moreaccessible for starch extraction.

[0092] This was unexpectedly achieved because of the fiber-likemorphology observed in the SEM micrographs. In blends containing morethan 45% by weight TPS, the starch phase has been completely extracted,that was an indication of a co-continuous morphology. Co-continuity isvery desirable for a maximum accessibility of the biodegradable portionin synthetic polymer/starch blends.

Tensile Properties

[0093] a) Machine Direction

[0094] LDPE/TPS blends were tested according to the ASTM D-638 method.Tensile specimens of type V were cut longitudinally from LDPE/TPSribbons. Samples were strained at 10 mm/min on a M30K machine (JJInstruments) equipped with a 5 kN cell and a data acquisition system.The average values of the Young's modulus, maximum tensile strength andelongation at break were calculated from at least 12 measurements.

[0095] The relative elongation at break (ε/ε₀) of LDPE/TPS blends ispresented in TABLE 2 and FIG. 6. In TABLE 2, ε, and ε₀ are theelongation at break of LDPE/TPS blends and pure LDPE, respectively.Blends containing high and intermediate glycerol contents maintain ahigh machine direction elongation at break, modulus and strength even athigh loading. In fact, the elongation at break of those blends arevirtually the same as the pure polyethylene. In ductile syntheticpolymer blends the high loading of an immiscible second phase results inhighly fragile materials. This occurs because elongation at break is aparameter which is highly sensitive to the state of the interface.Immiscible TPS/PE blends demonstrate high machine direction tensileproperties even in the absence of an interfacial modifier. Improvementin the elongation at break of these blends is an important featurecompared to prior art blends. This is probably due to the highlycontinuous nature of the dispersed TPS phase as well as the improvedremoval of water during processing. In the prior art methods, TPS wasblended with LDPE and then passed through the venting section. Since thedispersed TPS is encapsulated in an LDPE matrix, this led to impededwater removal. The presence of water at the processing temperature canproduce bubbles in the extrudate weakening the final product. In thepresent invention, water is extracted from TPS before mixing withpolyethylene and the problem of residual water is circumvented.

[0096] Blends having the lowest glycerol content failed at lowerelongation. This phenomenon was more marked in the case of blendsprepared with LDPE2049. The drop in the elongation at break of blendsprepared with TPS compounded with 20.0% glycerol was expected because ofthe larger size and poor dispersion of starch particles in the LDPEmatrix. The relative Young's modulus and maximum tensile strength ofLDPE (Ε₀) and LDPE/TPS blends (Ε) are shown in TABLE 2 and FIG. 7. Themodulus and maximum strength of LDPE/TPS blends compounded with highglycerol content decreases somewhat with TPS content. It is worth notingthat the 2040 LDPE/TPS blends with 27.5% glycerol maintain almost thesame machine direction modulus and maximum strength of polyethylene upto 35% TPS loadings. In contrast, the addition of TPS compounded with20.0% glycerol augmented the modulus of LDPE. That modulus was less thanfor LDPE/granular starch composites.

[0097] b) Cross Direction

[0098] Microtensile cross direction properties are shown in TABLE 3 forsamples conditioned for 48 hrs at 0% and 50% humidity. At 29% TPS, themodulus and maximum tensile strength are maintained at 80 and 83% thelevel of polyethylene at 0% humidity. At 50% humidity the modulus andmaximum tensile strength property retention is at 71 and 76%respectively. The elongation at break is diminished more significantly,but under all concentration conditions studied, the material remainshighly ductile. Thus, even the cross properties in this material performmuch better than that observed in typical immiscible synthetic polymerblends. It must be underlined that this blend material was prepared witha machine direction melt draw ratio of about 2:1. This results inpreferential dispersed phase orientation in the machine direction. Thusit is normal that the cross direction properties should be weaker. It ispossible to substantially improve the cross direction properties byminimizing the melt-draw ratio. Machine direction orientation can alsobe reduced by reducing the glycerol content. FIG. 2 showed lesselongation of the TPS phase in the md when 20% glycerol was used ascompared to the 27.5% case. For the case of 20% glycerol wheresignificantly less machine direction elongation was obtained, the crosselongation at break properties improve substantially as shown in TABLE3.

[0099] Table 3 clearly indicates that the cross direction modulus issubstantially increased at lower glycerol contents.

[0100] A number of parameters can be brought to bear in order to controlthe properties of the blend system. Applying an axial draw ratio can beused to modify the properties in the machine direction. Minimizing theaxial draw results in improved cross direction properties (particularlycross direction elongation at break). The system still maintains highcontinuity even under those latter conditions. Reducing the per-centglycerol results in an increase in the modulus of the blends. It ispossible using the above parameters to tailor the material to a givenapplication.

[0101] c) Effect of Aging

[0102] FIGS. 8-10 demonstrate the properties in the machine directionfor the 27.5% glycerol preparation. Two cases are shown: the propertiessoon after preparation (tested at about 50% humidity) and the propertiesafter one year (conditioned at 0% and 50% humidity for 48 hrs prior totesting). There is little effect of aging on the modulus, maximumtensile strength and elongation at break.

[0103] d) Effect of Humidity

[0104] In order to evaluate the effect of short term exposure tohumidity on mechanical properties, the samples were conditioned for 48hrs in 0% and 50% humidity environments as already mentioned above. Theresults for the machine direction properties are shown in FIGS. 8-10 forthe 27.5 glycerol preparations. For that glycerol preparation there isvery little effect of humidity on the machine direction elongation atbreak and maximum strength. A small effect of humidity is observed onthe modulus.

[0105] The cross direction properties shown in TABLE 3 demonstratesimilar tendencies as above for the 27.5% glycerol study. Very littleeffect of humidity is observed on the maximum strength and elongation atbreak. Some effect on the modulus is observed. TABLE 3 indicates thatfor the 20% glycerol preparation, humidity also results in a substantialincrease in the elongation at break. At 20% glycerol no effect ofhumidity is observed on the maximum strength. An effect on modulus isalso observed at the 20% glycerol concentration.

Transparency

[0106] One of the very particular features of the novel compositions ofthe present invention is that 1 mm thick ribbons of this blend with asmuch as 53% thermoplastic starch demonstrate a substantial level oftransparency.

[0107] Consequently, the results reported herein reveal LDPE/TPS blends,in sheet form, with high loadings of TPS that maintain essentially thesame elongation at break in the machine direction as pure PE even in theabsence of interfacial modifier(s). The modulus and maximum tensilestrength are also maintained at high levels. Good cross-properties arealso obtained. These blends are prepared in a combinedsingle-screw/twin-screw one-step process under carefully controlledprocessing conditions (flow rate, temperature, screw design,devolatization) and glycerol content. The morphology can be controlledby the composition and processing conditions to yield a highlycontinuous or co-continuous structure. In such a case, nearly all of theTPS becomes accessible for biodegradation.

[0108] In addition, the method of the present invention was also testedon blown film production. These experiments provided a film materialwhich exhibited a high level of transparency even at high loadings ofTPS.

[0109] Although the invention has been described above with respect tospecific embodiments, it will be evident to a person skilled in the artthat it may be modified and refined in various ways. It is thereforewished to have it understood that the present invention should not belimited in scope, except by the terms of the following claims. TABLE 2Mechanical properties of LDPE/TPS blends (ribbons) in the machinedirection. Glycerol TPS in slurry σ_(max) σ_(max)/ ε_(b) E Material (%)(%) (GPa) σ_(max0) (%) ε_(b)/ε_(b0) (GPa) E/E₀ a). LDPE2040 MFI = 20.0g/10 min. PE2040 0.0 0.0 11.8 1.00 482 1.00 55.9 1.00 P0A3 31.4 32.0 9.70.83 465 0.97 44.0 0.79 P0A4 46.2 32.0 8.7 0.74 449 0.93 44.4 0.79 P0A549.5 32.0 8.7 0.74 415 0.86 41.2 0.74 P0A13 29.0 27.5 10.5 0.89 464 0.9660.0 1.08 P0A14 35.5 27.5 9.8 0.83 427 0.89 50.1 0.90 P0A15 44.7 27.59.0 0.76 453 0.94 42.4 0.76 P0A16 53.3 27.5 8.2 0.69 388 0.80 34.2 0.61P0A23 29.8 20.0 9.8 0.83 400 0.83 60.2 1.08 P0A24 41.0 20.0 9.6 0.82 3450.72 64.4 1.15 P0A25 48.9 20.0 8.2 0.69 230 0.48 66.2 1.19 b). LDPE2049MFI = 12.0 g/10 min. PE2049 0.0 0.0 10.5 1.00 493 1.00 58.6 1.00 P9A332.1 32.0 8.9 0.85 492 1.00 41.5 0.71 P9A4 34.9 32.0 9.0 0.86 403 0.8241.6 0.71 P9A5 40.8 32.0 9.4 0.89 407 0.82 36.2 0.62 P9A13 33.7 27.5 9.00.86 480 0.97 40.5 0.69 P9A14 36.9 27.5 8.1 0.76 429 0.87 43.4 0.74P9A15 38.2 27.5 9.2 0.88 430 0.87 41.9 0.71 P9A23 28.6 20.0 6.3 0.60 890.18 68.7 1.17 P9A24 33.0 20.0 5.9 0.56 34 0.07 75.4 1.29 P9A25 46.320.0 6.1 0.58 65 0.13 71.8 1.23

[0110] TABLE 3 Mechanical properties of LDPE/TPS blends (micro-tensilespecimens) in the cross direction Glycerol TPS in slurry F_(max)F_(max)/ ε_(b) E Material (%) (%) (N) F_(max0) (%) ε_(b)/ε_(b0) (GPa)E/E₀ a). Conditioned at 0% relative humidity and room temperature.PE2040 0.0 0.0 26.5 1.00 220 1.00 43.3 1.00 P0A13 29.0 27.5 21.1 0.80 850.38 36.0 0.83 P0A14 35.5 27.5 19.1 0.72 62 0.28 35.0 0.81 P0A15 44.727.5 15.7 0.59 43 0.19 25.3 0.58 P0A16 53.3 27.5 13.8 0.52 33 0.15 24.50.57 P0A23 29.8 20.0 23.5 0.89 163 0.73 42.7 0.99 P0A24 41.0 20.0 22.60.85 84 0.38 40.7 0.94 P0A25 48.9 20.0 20.3 0.77 41 0.19 43.8 1.01 b).Conditioned at 50% relative humidity and room temperature. PE2040 0.00.0 25.9 1.00 328 1.00 44.1 1.00 P0A13 29.0 27.5 19.6 0.76 90 0.27 31.30.71 P0A14 35.5 27.5 18.1 0.70 71 0.22 27.0 0.61 P0A15 44.7 27.5 12.80.49 49 0.15 18.1 0.41 P0A16 53.3 27.5 9.3 0.36 39 0.12 14.0 0.32 P0A2329.8 20.0 24.0 0.93 454 1.38 33.8 0.77 P0A24 41.0 20.0 20.7 0.80 2560.78 30.8 0.70 P0A25 48.9 20.0 18.2 0.70 205 0.62 24.5 0.55

I claim:
 1. A method of preparing a thermoplastic starch and syntheticpolymer blend, said method comprising the steps of: (a) providing astarch suspension comprising starch, water and a plasticizer; (b)obtaining a thermoplastic starch from said starch suspension by causinggelatinization and plasticization of said starch suspension by exertingheat and pressure on said starch suspension in a first extrusion unit;(c) venting off residual water from said thermoplastic starch to obtaina substantially moisture-free thermoplastic starch; (d) obtaining a meltof a synthetic polymer or polymer blend in a second extrusion unit; (e)combining said melt obtained from step (d) with said substantiallymoisture-free thermoplastic starch obtained from step (c) to obtain athermoplastic starch and synthetic polymer blend.
 2. A method inaccordance with claim 1 wherein said plasticizer is glycerol.
 3. Amethod in accordance with claim 2 wherein said glycerol is present in aproportion of about 10 to about 32 wt % based on the total weight of theslurry suspension.
 4. A method in accordance with claim 1 wherein saidsynthetic polymer is polyethylene.
 5. A method in accordance with claim2 wherein said synthetic polymer is polyethylene.
 6. A method inaccordance with claim 3 wherein said synthetic polymer is polyethylene.7. A method in accordance with claim 1 wherein said first extrusion unitis a twin-screw extruder.
 8. A method in accordance with claim 7 whereinsaid second extrusion unit is a single-screw extruder.
 9. A method inaccordance with claim 1 wherein step (b) is conducted at a temperatureof about 50 to about 100° C.
 10. A method in accordance with claim 1wherein step (d) is conducted at a temperature of about 70 to about 200°C.
 11. A method in accordance with claim 1 wherein said starch is wheatstarch.
 12. A method in accordance with claim 8 wherein said single-screw extruder is connected to said twin-screw extruder at essentiallya right angle relative thereto.
 13. A composition of matter comprising aco-continuous blend of thermoplastic starch and synthetic polymer.
 14. Acomposition of matter comprising a highly continuous dispersed phase ofthermoplastic starch present in fiber-like strands in a primary phase ofsynthetic polymer.
 15. The composition of claim 14 wherein thermoplasticstarch (TPS) is present in a proportion of at least 20% by weight. 16.The composition of claim 14 wherein said thermoplastic starch is presentin a proportion of about 20 to about 80 wt %.
 17. The composition ofclaim 16 wherein said thermoplastic starch is present in a proportion ofabout 40 to about 60 wt %.
 18. The composition of claim 14 wherein saidthermoplastic starch comprises about 5 to about 50 wt % of aplasticizer.
 19. The composition of claim 18 wherein said plasticizer isglycerol.
 20. The composition of claim 14 wherein said synthetic polymeris polyethylene.
 21. The composition of claim 15 wherein said syntheticpolymer is polyethylene.
 22. The composition of claim 16 wherein saidsynthetic polymer is polyethylene.
 23. The composition of claim 17wherein said synthetic polymer is polyethylene.
 24. The composition ofclaim 18 wherein said synthetic polymer is polyethylene.
 25. Thecomposition of claim 19 wherein said synthetic polymer is polyethylene.26. The composition of claim 13 wherein said composition additionallycomprises a compatibilizer.
 27. The composition of claim 14 wherein saidcomposition additionally comprises a compatibilizer.
 28. The compositionof claim 13 wherein said thermoplastic starch is present in a proportionof about 20 to 80 wt %.
 29. The composition of claim 28 wherein saidthermoplastic starch is present in a proportion of about 40 to about 60wt %.
 30. The composition of claim 13 wherein said thermoplastic starchcomprises about 5 to about 50 wt % of a plasticizer.
 31. The compositionof claim 30 wherein said thermoplastic starch comprises about 10 toabout 35 wt % of a plasticizer.
 32. The composition of claim 31 whereinthe plasticizer is glycerol.